The present invention relates to a photo-sensor circuit for detecting a light signal and converting the detected light signal into an electric signal and a method of operating the circuit, and, more specifically, to a photo-sensor circuit having a shutter function.
FIG. 1 shows a photo-sensor circuit having a shutter function (sample-and-hold function), which can be used as one of pixel detecting elements composing an image sensor. This photo-sensor circuit comprises a photodiode PD for detecting a light signal and converting it into an electric signal, a MOS transistor Q1 for charging/discharging a capacitor C1 being a parasitic capacitance of the photodiode, a capacitor C2 for accumulating a terminal voltage of the photodiode PD as a pixel signal, a MOS transistor Q2 for transferring an electric charge from the capacitor C1 to the capacitor C2, a MOS transistor Q3 for amplifying a terminal voltage of the capacitor C2 and a MOS transistor Q4 for selectively outputting an amplified pixel signal.
The above conventional photo-sensor circuit works with signals generated by respective parts at respective timings as shown in FIG. 9 when operated by a conventional method.
Namely, a transistor Q1 is turned ON when a driving voltage V1 increases to a high level at timing t1–t2 to give an electric charge to the capacitor C1 being a parasitic capacitance of the photodiode PD. Once the photodiode PD is illuminated, a sensor current flows causing the capacitor C1 to reduce the electric charge by an amount proportional to the flowing current.
The transistor Q2 is then turned ON when a driving voltage V3 increases to a high level at timing t3–t4 to transfer an electric charge from the capacitor C1 to the capacitor C2. When the transistor Q4 is then turned ON by an increase in driving voltage V4 to a high level at timing t4–t5, a current from a power supply V5 is supplied and limited by the transistor Q3. Consequently, a pixel signal Vout is output through a resistance R.
In this photo-sensor circuit, the transistor Q2 becomes turned-off at timing t4 and the capacitor C2 maintains a constant electric charge until the transistor Q2 is turned OFF to transfer the electric charge from the capacitor C1 to the capacitor C2. In other words, during the turned-off period of the transistor Q2 (a holding time of the capacitor C2), the same output can be obtained as a pixel signal independent of a change in the terminal voltage Vc1 of the capacitor C1.
Owing to the above-described structure, the photo-sensor circuit can act as a shutter for a single pixel. The open time of this shutter can be controlled.
FIG. 10 shows another structure of the photo-sensor circuit having a shutter function, wherein a MOS transistor Q5 is further provided for charging and discharging the capacitor C2.
The operation of the thus constructed photo-sensor circuit differs from that of the fore-mentioned photo-sensor circuit by the fact that its pixel signal is initialized by discharging the capacitor C2 when the transistor Q5 of the circuit is turned ON by an increase in driving voltage V6 at timing t6–t7 as shown in FIG. 11.
The photo-sensor circuit shown in FIG. 1 can transfer a terminal voltage Vc1 of the capacitor C1 to a capacitor C2 by the action of the transistor Q2 and can retain the electric charge on the capacitor C2 until the transistor Q2 is turned ON again. Consequently, in case there is a difference between the terminal voltages Vc1 and Vc2 of the capacitors C1 and C2, the terminal voltage Vc1 of the capacitor C1 is not correctly reflected on that of the capacitor C2 until the transistor Q2 is turned ON again. This results in decreasing the reproducibility of the signal.
FIG. 12 shows a model of accumulation of electric charge in the capacitors C1 and C2 respectively while the photo-sensor circuit of FIG. 1 is operated by a conventional method and is continuously sensing light signals.
In the photo-sensor circuit constructed as shown in FIG. 10, the capacitor C2 can be charged and discharged by the transistor Q5 and hence the terminal voltage Vc1 of the capacitor C1 can be well-reproducibly transferred to the capacitor C2. However, there arises such a problem that a charge of the capacitor C2 becomes smaller than a charge of the capacitor C1.
FIG. 13 shows a model of accumulation of electric charge in the capacitors C1 and C2 respectively while the photo-sensor circuit of FIG. 10 is operated by a conventional method.